Field of the Invention
The invention relates to a method of minimizing trapping, i.e., choking or spreading, in a printing-original production process, in particular on printing copies with different colored boundary areas.
The published German Patent Document DE 196 38 967 C2 is concerned with a group of measurement fields and a method of registering optically detectable printing-technology variables in multicolor production printing or final printing runs. The groups of measurement fields are optically scannable onto a multicolored printed sheet with a predefined structure by a printing machine. For each measurement field, at least one measurement-field map exhibits a specific rotation relative to the printing direction. In order to determine at least one optical printing variable, each measurement field has a minimum dimension which is sufficiently large, in terms of area, for allowing a measurement of the area coverage also to be made. Assigned to the measurement fields is at least one narrow strip in the print, which extends at a relatively small distance from the edge of the measurement field. The desired or nominal area, which is to remain free of ink in the same print, and which is located between the edge of the measurement field and the strip, can be compared with the corresponding actual area.
U.S. Pat. No. 4,583,116 is concerned with a method of registering a multiplicity of printing parameters. In performing the proposed method of this reference, an effort is made to compensate, in multicolor printing, for the errors which occur when superimposing the respective color separations for magenta, cyan, yellow and black. Within the printed image, a section is defined, within which the color separation signal determining the contour of the region is determined. Depending upon the color separation signals which are obtained, the boundaries of regions of mutually overlapping colors are shifted in relation to one another in accordance with the color separation signal determined to be predominant.
In addition, attempts have been made to determine the spreading widths automatically in order to generate spreading frames on colored edges on a printed page. A test plate with an arrangement of test elements was printed, the test elements containing a number of test element patterns with predefined desired positions. The printed test plates are scanned and the image data of the test elements are stored. By analyzing the image data test elements, the actual positions of the test element patterns and their deviations from the desired positions are determined. For two color separations in each case, the shifts in the test element patterns in relation to one another are determined, an optimum spreading width between the color separations being calculated from the shifts.
In the preprinting stage, in order to compensate for register differences in the print, trapping, i.e., choking and spreading, must be applied in printing originals. This choking or spreading is used in the color separation if different colored objects adjoin one another in the subsequent print. In order to avoid so-called “flashes”, which can appear as white transparently between adjacent colored areas in the printing process, colored areas have to be enlarged, i.e., spread, or reduced, i.e., choked. The objective of this procedure is, for boundary areas, to print the individual color separations over one another to such an extent that, if there are local deviations of the color separations from one another, no unprinted paper area (flashes) remains visible between the objects. The size of the overlap must ensure that the maximum register differences that occur are covered. On the other hand, in case overlaps are made too large, the dependence of the color areas printed on one another can cause a reinforced visual appearance of the object boundaries. One speaks of the occurrence of so-called “dirty edges”, which is likewise undesired on the finished print.
The intensity of the spreading or choking must therefore be selected as large as required based upon the register differences, but on the other hand kept as small as possible.